1. Field of the Invention
The present invention relates generally to a rotary hydraulic pressure control valve which controls a hydraulic pressure through a coaxial relative angular displacement between a valve body and a valve spool, and more particularly to a rotary hydraulic pressure control valve which controls a hydraulic pressure delivered to a steer-assisting hydraulic cylinder in accordance with the operation of a steering wheel.
2. Description of Related Art
There is known a hydraulic pressure steering apparatus which assists steering with a hydraulic pressure generated by a double-acting hydraulic cylinder provided in a steering mechanism, thereby saving the labor required in steering and giving a smooth steering feeling. The known hydraulic steering apparatus is provided with a hydraulic pressure control valve provided among a pair of cylinder chambers (to which oil is transferred) housing a hydraulic cylinder, a hydraulic pump (hydraulic pressure source) driven by an engine, and an oil tank (into which the working oil is drained). The hydraulic pressure control valve controls the delivery and drainage of the working oil in accordance with the amplitude and direction of a torque exerted on the steering wheel.
The known hydraulic pressure control valve generally includes a rotary valve designed to utilize the rotation of the steering wheel. This known control valve is provided with an input shaft connected to the steering wheel and an output shaft connected to the steering mechanism, wherein the input shaft and the output shaft are coaxially connected by means of a torsion bar. A cylindrical valve body is joined to one end of the connected input-output shaft, and a valve spool joined to the other end thereof is coaxially fitted in the valve body such that a relative rotation of the valve body or the valve spool is possible. When a torque is exerted on the steering wheel, the torsion bar is twisted to cause a relative angular displacement to occur between the valve body and the valve spool.
The valve body and the valve spool are provided with oil grooves axially cut in the peripheral inside and outside surfaces, respectively, so as to enable a working oil to flow therethrough. The oil grooves of the valve body and valve spool are circumferentially arranged in zigzag positions with lands interposed between circumferentially adjacent oil grooves. The adjacent oil grooves of the valve body and valve spool communicate with each other, and each oil groove and land opposite thereto define an oil supply chamber connected to an oil supply source, an oil drain chamber connected to an oil drain tank, and an oil transfer chamber connected to a cylinder chamber through which oil is transferred to a hydraulic cylinder.
In FIGS. 1A and 1B, which are views exemplifying the operation of a known hydraulic pressure control valve, the valve body 1 and valve spool 2 are shown in an elevation, particularly to show the opposing inside and outside walls of them. As referred to above, the oil grooves 4 of the valve body 1 constitute oil transfer chambers 12 and 13 against the opposing lands along the peripheral surfaces of the valve body 1, each of the oil transfer chambers being connected to cylinder chambers S.sub.R and S.sub.L of a hydraulic cylinder (not shown) through respective bores. The oil grooves 5 of the valve spool 2 are arranged on and along the peripheral outside surface of the valve spool 2 constitute oil supply chambers 10 connected to a hydraulic pressure pump P (oil supply source) through supply bores, oil drain chambers 11 connected to an oil drain tank T through drain bores. The oil supply chambers 10 and oil drain chambers 11 are alternately arranged. It is also possible that the oil grooves 4 of the valve body 1 constitute the oil supply chambers 10 and oil drain chambers 11, and the oil grooves 5 of the valve spool 2 constitute the oil transfer chambers 12 and 13 against the opposing lands.
The oil grooves 4 and 5 communicate with each other through gaps having the same extent of openness on both sides as shown in FIGS. 1A and 1B. These gaps function as throttles whose extent of openness is changeable in accordance with a regular angular displacement occurring between the valve body 1 and the valve spool 2. Hereinafter, these gaps will be referred to as "throttle(s) 6" and the space (extent of openness) of the gap as "throttling extent". Thus, the hydraulic pressure delivered to the cylinder chambers S.sub.R and S.sub.L through the oil transfer chambers 12 and 13 is controlled in accordance with changes in the throttling extent.
FIG. 1A shows a state where no relative angular displacement occurs between the valve body 1 and the valve spool 2. The oil from the pump P is evenly supplied to the oil transfer chambers 12 and 13 because of the equal throttling extent of the throttles 6 on both sides of the oil supply chambers 10, and is delivered to the oil drain chambers 11 through the other throttles 6. In this way, the working oil to be supplied to the oil supply chamber 10 is prevented from flowing into either of the cylinders S.sub.R and S.sub.L. No hydraulic power is generated by the hydraulic cylinder.
FIG. 1B shows a state where a torque is exerted on the steering wheel to cause a relative angular displacement to occur between the valve body 1 and the valve spool 2. In this case, one of the throttles 6 (toward the oil transfer chamber 12) on both sides of each oil supply chamber 10 is widened, and the other (toward the oil transfer chamber 13) is restricted. As a result, most of the working oil is flown into the oil transfer chamber 12 through the wider throttle 6, thereby producing pressure difference between the oil transfer chambers 12 and 13, and between the cylinder chambers S.sub.R and S.sub.L connected thereto. The hydraulic cylinder generates hydraulic pressure corresponding to the pressure difference (steer-assisting power).
The pressure difference depends upon the degree of restriction occurring in the other throttles 6 (toward the oil transfer chamber 13), and the degree of restriction depends upon the amplitude of the relative angular displacement between the valve body 1 and the valve spool 2 which depends upon the amplitude of the torque exerted on the steering wheel. As a result, the steer-assisting power generated by the hydraulic cylinder has a direction and amplitude corresponding to the torque exerted on the steering wheel, thereby assisting the operation of the steering wheel. The oil forced out through the other cylinder chamber S.sub.L in accordance with the operation of the hydraulic cylinder is returned to the oil transfer chamber 13, and is introduced into the adjacent oil drain chamber 11 through the widened throttles 6 (toward the oil drain chamber 11). Then the oil is drained into the oil drain tank T which is connected to the oil drain chamber 11.
The corners of the oil grooves 5 of the valve spool 2 facing the respective throttles 6 are circumferentially chamfered to produce chamfered portions 7. More specifically, the corner that the side of each oil groove 5 and the top surface of each land meet is diagonally cut at a given angle so as to be flat in the cut-away width. The chamfered portions 7 are helpful in causing gradual changes in the throttling extent in the throttles 6. A desired increase in the steer-assisting power is not a proportional increase to the torque exerted on the steering wheel but a gradual increase in the range of small torques and a rapid increase beyond a predetermined limit. This desired characteristic is obtained by providing the chamfered portions 7.
The above-mentioned hydraulic pressure control valves are disadvantageous in that when a large relative angular displacement occurs between the valve body 1 and the valve spool 2, the passage of the oil through the restricted throttles 6 involves the occurrence of cavitation, thereby causing harsh noise. The harsh noise is unpleasant and gives strain to the driver.
The widening and restricting of the throttles 6 occur oppositely between both sides of each oil supply chamber 10 and of each oil drain chamber 11. More specifically, each oil supply chamber 10 is adjacent to the throttles 6 toward the oil transfer chambers 12 on one side and the oil transfer chamber 13 on the other side. In the situation shown in FIG. 1B the former throttle 6 is widened and the latter throttle 6 is restricted. On both sides of each oil drain chamber 13 quite the opposite action occurs, that is, the throttle 6 toward the oil transfer chamber 13 is widened, and the throttle 6 toward the oil supply chamber 12 is restricted.
The harsh noise referred to above occurs when the working oil flows through the restricted throttles 6. It has been found out that cavitations accompanied by the oil flow are different with the flows. Taking advantage of this discovery various types of hydraulic pressure control valves have been proposed for reducing harsh noise.
Referring to FIGS. 1A and 1B, one example disclosed in Japanese Utility Model Publication No. 1-43974 (1989) will be described:
It was found out that noise due to cavitations occurs remarkably in the throttles 6 on both sides of the oil drain chambers 11 more often than in those on both sides of the oil supply chamber 10. FIGS. 2A and 2B show an example in which the chamfered portions 7 at the throttles 6 on both sides of the oil supply chamber 10 are formed at a larger angle than those on both sides of the oil drain chamber 11 are. Thus noise is reduced.
In the example of FIGS. 2A and 2B, when a relative angular displacement of the valve spool 2 occurs against the valve body 1, the throttle 6 on one side (toward the oil transfer chamber 13) of the oil supply chamber 10 has a wider throttling extent than the throttle 6 on the same side (toward the oil transfer chamber 12) of the oil drain chamfer 11, and the oil flow, which otherwise would cause harsh noise, is concentrated in the throttle 6, thereby reducing the occurrence of noise.
FIGS. 3A and 3B show another example disclosed in Japanese Patent Publication Laid-Open No. 6-206555 (1994). This example has substantially the same structure as that of Japanese Utility Model Publication No. 1-43974 (1989) but is different in that the chamfers 7 at the corners on the side of the valve body 1 in the throttles 6 on both sides of the oil drain chamber 11 are formed so as to reduce the occurrence of noise.
Other example disclosed in Japanese Patent Application Laid-Open No. 6-156292 (1994). This example has substantially the same structure as that of the above-mentioned two examples but is different in that the chamfers 7 at corners on the side of the valve body 1 in the throttles 6 on both sides of the oil supply chamber 10 are formed, since those throttles 6 are disadvantageous to cavitation.
Utility Model Publication No. 54-28735 (1979), Patent Publication Laid-Open 60-203580 (1985) and U.S. Pat. No. 3,022,772 show a further example, as shown in FIGS. 4A and 4B, in which the chamfered portions 7 are provided in the throttles 6 on both sides of the oil supply chamber 10 so as to reduce the occurrence of noise. This is based on the discovery that noise due to cavitation occurs in the throttles 6 on both sides of the oil drain chambers 11 more often than in those on both sides of the oil supply chambers 10 under the structure in which the oil grooves 4 of the valve body 1 constitute the oil supply chambers 10 and the oil drain chambers 11, and the oil grooves 5 of the valve spool 2 constitute the oil transfer chambers 12 and 13.
According to this example, as shown in FIG. 4B, the throttle 6 on one side (toward the oil transfer chamber 13) of the oil drain chamfers 11 having no chamfered portion is restricted as soon as a relative angular displacement of the valve spool 2 occurs for the valve body 1, whereas the throttles 6 having the chamfered portions 7 on the same sides (toward the oil transfer chamber 12) of the oil supply chambers 10 have wider throttling extent. As a result, the working oil is concentrated in the wider throttle 6 between the oil supply chamber 10 and the oil transfer chamber 12, thereby reducing the occurrence of noise.
The above-mentioned hydraulic pressure control valve seems to have substantially the same function as that of the hydraulic pressure control valve disclosed in Utility Model Publication No. 1-43974 (1989) and shown in FIGS. 2A and 2B. However, looking at the flow state disclosed in Japanese Patent Application Laid-Open Nos. 6-206555 (1995) and 6-156292 (1995) (Refer to FIG. 3A and 3B), the flow state in this hydraulic pressure control valve (Refer to FIGS. 4A and 4B) is same as that disclosed in Japanese Patent Application laid-Open No. 6-156292 (1995) and opposite to those disclosed in Japanese Patent Application Laid-Open No. 6-206555 (1995) and Utility Model Publication No. 1-43974 (1989).
In summary, the known hydraulic pressure control valves aim at eliminating the possibility of noise due to cavitations by recognizing differences in the manner in which cavitation occurs but the contents of the recognition are different from one after another.
Experiments were conducted according to the present invention to define the manner in which cavitation occurs. A oil path mimically structure linearly between the oil supply chamber 10 and the oil drain chamber 11 is constructed so as to be visible from the outside. The manner in which cavitation occurred in case where the throttling extent in the oil supply chamber 10 was restricted, was compared with the manner in which cavitation occurred in case where the throttling extent in the oil drain chamber 11 was restricted, using data measuring noise and observing by eye measurement.
FIGS. 5 and 6 show the results obtained through experiments conducted according to the present invention. FIG. 5 shows that the oil supply chamber 10 and the oil drain chamber 11 are provided on the side of the valve spool 2, as shown in FIGS. 1A, 1B, 2A, 2B, 3A and 3B, and FIG. 6 shows that the oil supply chamber 10 and the oil drain chamber 11 are provided on the side of the valve body 1, as shown in FIGS. 4A and 4b. The results of measuring noise are depicted in FIGS. 5 and 6, with the horizontal-axis representing hydraulic pressures (kgf/cm.sup.2) in the up-stream, and with the vertical-axis representing the level (dB) of noise measured. The results of observing cavitations are indicated in terms of comparative assessment based on visual observations.
In Case (a) of FIG. 5 the throttling extent on the side of the oil supply chamber 10 are restricted, and in Case (b) those on the side of the oil drain chamber 11. The comparison indicates that a clear difference appears in the measuring data in a region where hydraulic pressure up-stream of the throttle is below 60 kgf/cm.sup.2, and especially in Case (a) the level of noise are kept at such a low level as 20 dB or so in a region where hydraulic pressure up-stream of the throttle is below 40 kgf/cm.sup.2. In Case (b), it will be recognized that in a region where the hydraulic pressure up-stream of the throttle exceeds 20 kgf/cm.sup.2 noise occurs which has high level (40 dB or so) equal to the high-pressure region.
It was visually observed that in Case (a) no cavitation was observed so long as the throttling extent were small and the hydraulic pressure up-stream of the throttle remained at low pressure, and that in Case (b) cavitation was already observed when the throttles had relatively wide throttling extent. It will be understood from this that in the structure shown in FIG. 5, in which the oil supply chamber 10 and the oil drain chamber 11 are provided in the valve spool 2, the flow in the throttle on the side of the oil supply chamber 10 is less contributive to the occurrence of cavitation.
FIG. 6 shows opposite cases to those of FIG. 5, that is, Case (a) in which the throttles on the oil drain chamber 11 are restricted, and Case (b) in which the throttles on the oil supply chamber 10 are restricted. The comparison of data in Cases (a) and (b) indicates that there is a difference from the cases of FIG. 5 in the region where the hydraulic pressure up-stream of the throttles is below 30 kgf/cm.sup.2. Case (a) shows that the noise is at low level. It was visually confirmed that the occurrence of cavitation in Case (a) is less frequent than that in Case (b). It will be understood from this that in the structure of FIG. 6, in which the oil supply chamber 10 and the oil drain chamber 11 are provided in the valve body 1, the flow in the throttle on the oil drain chamber 11 is less contributive to the occurrence of cavitation.
In the light of the experimental results mentioned above, the known hydraulic pressure control valves disclosed in Utility Model Publication No. 1-43974 (1989), Patent Publication Laid-Open No. 6-206555 (1944) correspond to the experimental results, and those disclosed in Utility Model Publication No. 54-28735 (1979), Patent Publication Laid-Open No. 60-203580 (1985), Patent Publication Laid-Open No. 6-156292 (1994), and U.S. Pat. No. 3,022,772 are contrary to the experimental results and ineffective for reducing the occurrence of noise.
The chamfered portions 7 are generally provided in the corners of the valve spool 2 but there are some which are provided in the corners of the valve body 1. The occurrence of cavitation in these instances corresponds to the throttles appearing upside down shown in FIGS. 5 and 6. It will be understood from FIGS. 6 and 5, respectively, that in the structure in which the oil supply chamber 10 and the oil drain chamber 11 are provided in the valve body 1, the throttles on the oil drain chamber 11 are less contributive to the occurrence of cavitation, and that in the structure in which the throttles are provided in the oil supply chamber 10 and the oil drain chamber 11, the throttles on the oil supply chamber 10 are less contributive to the occurrence of cavitation. This indicates that when the chamfered portions 7 are provided in the valve body 1, the structures disclosed in Utility Model Publication No. 1-43974 (1989) and Patent Publication Laid-Open No. 6-206555 (1994) are rather contributive to the occurrence of cavitation.
Each of the proposals for reducing noise is only applicable to the respective specific structure including the oil supply source, the oil drain tank and the oil transfer chambers, the oil grooves of the valve body 1 and the valve spool 2, the chamfered portions in the throttles, and the combination thereof but there is no proposal which is applicable to every type of hydraulic pressure control valve.
As shown in FIGS. 4A and 4B the throttles having no chamfered portions is not desirable for obtaining sufficient steer-assisting power. In order to secure an effective amount of steer-assisting power, each of the oil drain chambers 11 is provided with the chamfered portions 7 having a narrower width than that of the chamfered portions 7 on both sides of the oil supply chamber 10 and the throttles 6 on both sides of the oil drain chamber 11 are restricted earlier than those on both sides of the oil supply chamber 10.
However, in the structure mentioned above a difference in the width of the chamfered portion 7 required for actual use is such a small amount as 0.05 mm as to achieve. In addition, two kinds of chamfered portions 7 differing in width are alternately provided, which requires many steps of operations and highly technical skill.